Formerly one had clamped steel springs, predominantly used as a holding-down apparatus, in the configuration of coil springs and dish springs between the pressure plate of the deep-drawing press and the holding plate. Also, rubber springs found useage. All of these springs have the common disadvantage that their spring force increases relatively steeply in the course of the spring movement. The corresponding spring characteristic curves are schematically illustrated in FIG. 1, and indeed as characteristic curve "A" for coil springs and as characteristic curve "B" for dish springs. Further explanation of the FIG. 1 illustration will given at the beginning of the exemplary embodiment.
An optimal deep-drawing operation only comes into practice if one apportions the holding-down force correctly to the material and correctly to the work tool and if this force remains constant during the entire deep-drawing process, thus having a characteristic curve I parallel to the abscissa in the context of the FIG. 1 illustration. This standard cannot be achieved with steel-or rubber springs, because their characteristic curves intersect the ideal characteristic curve I at a single point; the holding-down force practised by them remains too small on the spring curve lying before this point and is too large after this ideal point. If one would adjust the springs without pre-tension, then the holding-down force practised by them would equal zero at the beginning of the deep-drawing process.
A further disadvantage of the known holding-down apparatuses comprised of spring elements occurs by the short operative spring path which is still shorter due to the absolutely required pre-tension, compare FIG. 1. One is obliged on account of the short spring path to execute the deep-drawing process in several draws, in order to obtain a flat workpiece.
In recognition of this state of affairs one has in recent years desired to develop gas springs which have a flat characteristic curve. One such pneumatic holding-down apparatus has been very well known on the market. It comprises a plurality of piston cylinders whose cylindrical chambers communicate with a large pressure container. Piston cylinder and pressure container are filled with nitrogen at high pressure of 100 bar in the overall arrangement. The entire volume of all piston cylinders should be considerably smaller than the volume of the pressure container arranged to them. Under this premise a plunging of all pistons into the piston cylinders changes the total pressure relatively slightly. If the piston cylinders are positioned as holding-down apparatuses between a pressure plate and a holding plate, then the gas resistance remains substantially constant opposite the piston movement, assuming that the container volume is sufficiently much larger than the sum of the cylinder volumes.
The characteristic curve of this known nitrogen-spring element extends undisputably at a very sharp angle to the coordinate axes of the spring movement. This characteristic curve is identified in FIG. 1 with "C" and cuts the ideal curve I at the point I.sub.C. Its inclination in relationship to the ideal curve is according to the technical journal "Strips, Sheets, Pipes", page 449 (1974) about 10%, i.e., corresponding to an angle of about 6 degrees.
An obvious difficulty of this system concerns the very high pressure required also outside the piston cylinder. The large volume pressure container as well as the pressure tube leading from this container to the piston cylinders must be dimensioned according to safety reasons and are therefore also corespondingly expensive. Besides it is complicated and expensive for a system to have to use a special apparatus which must have highly compressed nitrogen in storage.
The operation of the known nitrogen gas spring is controlled by the Boyle-Mariotte Law. This law is however related to an ideal gas and is valid only with limitations for a real gas such as nitrogen. A useful approximation assumes for example that the gas at high pressure has the so-called Boyle temperature which is a predetermined specific temperature for each gas. For nitrogen the Boyle temperature amounts to 56.degree. C.
During the deep-drawing process the temperature of the nitrogen provided in a piston cylinder climbs quickly to a very high value. The nitrogen located in the pressure container remains by contrast at room temperature, on account of the slight heat conductivity of the gas the heat produced in the piston cylinder cannot be transmitted through the nitrogen in the pressure container because no flow is provided in the closed system. The both Boyle-Mariotte volumes work in sequence with widely different temperatures. The result is that one must hold the relationship pressure container -- room content/piston cylinder volume very large in order to guarantee the correctness of the Boyle-Mariotte Law with sufficient approximation.
Probably on cost-and safety grounds one in the art goes another way: one constructs the pressure container of the nitrogen gas-spring element on a so-called base plate on which the spring elements are mounted as holding-down elements. By this measure one avoids the heavy and expensive high pressure tubes, however it therefore naturally acquires an invalid volume relationship and a correspondingly steep characteristic curve. A further disadvantage is that the deep-drawing process by this arrangement can only be performed from below upwardly, and therefore is not operative by a plurality of presses.